The heaviest users of the open-source operating system came from the Unix world, where the migration decisions were much easier because Linux environments are similar, skill sets transfer well, and developers can keep the same tools. Financial institutions especially also relished the chance to save substantial sums of money by moving to commodity hardware.

Yet even that group isn't abandoning Unix. Organizations don't walk away from years of investment, says Dan Kusnetzky, an analyst at IDC. The research firm's studies show that companies generally bring in Linux alongside their Unix servers, he says.

Fidelity Investments, for example, moved some of its Oracle databases from Sun Solaris boxes to Intel-based Linux servers because it enabled the company to merge database instances and reduce database licenses, according to Donald Haine, the former CIO and now a venture partner at the Boston-based financial services firm. But Haine predicts that the transition to Linux may slow now that new capabilities in Solaris 10 will enable Fidelity to achieve the server consolidation it seeks in its Sun environment.

"Why should I take the risk of moving some fairly critical applications when now I can do what I want to do?" Haine says. "I've had a huge investment in my [Sun-based] applications, and people get nervous when you move onto a new platform."

With even Unix users cautious about making a large-scale switch, it's not surprising to find that it's more difficult to ferret out IT shops that have made a major shift from Windows to Linux. Migrations from Windows to Linux happen on a limited scale with new Web applications. But they're hardly commonplace.